Cellular communication system is a communication system in which a base station carries out radio communications with a plurality of communication terminals simultaneously, accompanying with recent increasing demands that is requested to increase the transmission efficiency thereof.
HDR (High Data Rate) has been proposed as a technique to increase transmission efficiency of the downlink channel from base station to communication terminal. The HDR is a method of transmitting data thereby the base station carries out scheduling in which communication resources are time-divided and assigned to each communication terminal, further, the base station sets a transmission rate to each communication terminal according to downlink channel quality.
Now, a description will be made to the operation in which a base station and a communication terminal carry out HDR radio communication. First of all, the base station transmits a pilot signal to each communication terminal. Each communication terminal measures the downlink channel quality expressed by CIR (desired signal carrier to interference signal ratio), etc., based on the pilot signal and obtains a transmission rate that permits the communication therebetween. Then, each communication terminal selects a communication mode, which is a combination of a packet length, coding method, modulating method and spreading factor, based on the transmission rate that permits the communication therebetween, and transmits to the base station a data rate control (hereinafter, referred to as “DRC”) signal which represents the communication mode.
The kinds of selectable modulating method in each system are predetermined such as BPSK, QPSK, 16QAM, 64QAM, etc. Also, the kinds of selectable coding in each system are predetermined such as ½ turbo coding, ⅓ turbo coding, ¾ turbo coding, etc. Furthermore, the kinds of selectable spreading factor in each system are predetermined such as 64 times spreading, 128 times spreading, 256 times spreading, etc. Still further, a plurality of selectable transmission rates are predetermined depending on the combination of a packet length, modulating method, coding method and spreading factor. From those combinations, each communication terminal selects a combination which permits the most efficient communication under the present downlink channel quality, and transmits a DRC signal which represents the selected communication mode to the base station. In general, the DRC signals are represented by numbers from 1 to N; the larger number represents the higher downlink channel quality.
The base station carries out the scheduling based on the DRC signal transmitted from each communication terminal, and notifies each communication terminal via a control channel with an information that indicates an assignment of communication resources for each communication terminal.
Generally, while taking into consideration an increase of the transmission efficiency of the system, the base station assigns the communication resources to a communication terminal having a higher downlink channel quality and giving priority thereto. Indeed, the base station assigns a larger number of time slots per one frame to a communication terminal having the higher downlink channel quality.
As described above, in a conventional HDR, the data transmission efficiency of an entire system is increased by assigning communication resources to each communication terminal based on the channel quality.
In the aforementioned conventional HDR, since the assignment of time slots is determined only based on the channel quality, the number of time slots to be assigned per one frame is reduced in a communication terminal having a poor channel quality. That is to say, in a communication terminal having a poor channel quality, the interval between the time slots assigned thereto becomes large.
In a communication terminal having a poor channel quality, since the interval between the assigned time slots is large, it takes a long time up to termination of the data communication in spite of the fact that almost all of data has been received. For example, it is conceivable that, at a point of time when a reception of all data for a tune of music has almost completed, it takes a long time up to the point of time when the data of the last slot is received, etc. Accordingly, in such a communication terminal, a waiting time becomes long resulting in an increase of power consumption in spite of the fact that the data communication has almost completed.
Moreover, at a time when almost all of data have been received, if the data communication with a communication terminal is terminated, it is possible to assign the time slots which have been assigned to the communication terminal to another communication terminal. Accordingly, from the viewpoint of the transmission efficiency in the entire system, it is inefficient that the interval of the time slots assigned to a communication terminal which is at the point of time when the data communication has almost completed is large.